Assuming that a plurality of antenna ports (AP: Antenna Port) of a base station is arranged in a transverse direction, a technique for executing beam forming in the horizontal direction has been adopted for Releases 8 to 11 of the Third Generation Partnership Project (3GPP) standards.
For Release 12 of the 3GPP standard, three-dimensional MIMO (3D-MIMO: Three Dimensional Multiple Input Multiple Output) has been studied such that multiple antenna elements, which are two-dimensionally arranged in the vertical direction and horizontal direction, are installed in a base station, and a beam is formed in the vertical direction, in addition to the horizontal direction. It can be expected that system characteristics can be enhanced by forming a beam in the vertical direction and horizontal direction.
For purposes of 3GPP standardization, the 3D-MIMO for a case where the number of the antenna ports is less than or equal to 8 is referred to as vertical beam forming, and the 3D-MIMO for a case where the number of the antenna ports is greater than 8 (e.g., 16, 32, 64, . . . ) is referred to as Full Dimension-MIMO (FD-MIMO). FD-MIMO is often referred to as “Massive MIMO.”
Massive MIMO can improve frequency utilization efficiency by forming a sharp beam by using a great number of antenna elements of a base station.
Furthermore, reference signals (CSI-RS: Reference Signal for CSI measurement) for measuring channel state information (CSI: Channel State Information) for a case where the number of the antenna ports is less than or equal to 8 has been specified in Release 10 of the 3GPP standard (cf. 3GPP TS 36.211 V10.7.0 (2013-02) Sec 6.10.5 and 3GPP TS 36.331 V10.0.0 (2013-06) Sec 6.3.2). FIG. 1 shows an example of mapping of the CSI-RS for the case where the number of the antenna ports is less than or equal to 8. To reduce the overhead of the CSI-RS, in a frequency domain, one resource element (RE: Resource Element) is allocated per one antenna port in one resource block (RB: Resource Block). Additionally, in a time domain, the CSI-RS is transmitted with a transmission period of 5, 10, 20, 40, or 80 milliseconds. The transmission period of the CSI-RS is set by Radio Resource Control (RRC) signaling.
To signal, to a mobile station, the mapping of the CSI-RS onto resource elements within a resource block, a table (CSI-RS configuration) that is shown in FIG. 2 is used (cf. Table 6.10.5.2-1 of Sec 6.10.5 of 3GPP TS 36.211 V10.7.0 (2013-02)). FIG. 2 shows an example of a table that is used for specifying a resource configuration of the CSI-RS.
For example, for a case where the number of the antenna ports is 2, there are twenty types of CSI-RS mapping, which are shown in (A) of FIG. 1. To signal, to a mobile station, which one of the twenty types of the mapping is to be used, one of the indexes from 0 to 19 (CSI reference signal configuration) in the table of FIG. 2 is used.
Additionally, to signal, to a mobile station, the transmission period of the CSI-RS and a start position of a subframe (a subframe offset), a table that is shown in FIG. 3 (CSI-RS subframe configuration) is used (cf. Table 6.10.5.3-1 of Sec 6.10.5 of 3GPP TS 36.211 V10.7.0 (2013-02)). FIG. 3 shows an example of a table that is used for specifying a subframe configuration of the CSI-RS.
The CSI-RS that is specified by using the table of FIG. 2 is transmitted with a period of 5, 10, 20, 40, or 80 milliseconds, while the CSI-RS is multiplexed in the subframe. To specify the period and the start position of the subframe, one of the indexes from 0 to 154 in the table of FIG. 3 (CSI RS-SubframeConfig) is signaled to the mobile station.